Hi Dan,
I don't know about a DSP radio but I do know that if you modulate a regular
old fashion SSB rig with a square wave you will not get anything out of it
at RF that resembles a square wave. Looking at it in the time domain.
73
Gary K4FMX
> -----Original Message-----
> From: Dan Mills [mailto:dmills@exponent.myzen.co.uk]
> Sent: Monday, January 02, 2012 7:24 PM
> To: garyschafer@comcast.net
> Cc: amps@contesting.com
> Subject: RE: [Amps] IMD
>
> On Mon, 2012-01-02 at 16:31 -0500, Gary Schafer wrote:
> >There is no easy way to do audio clipping for SSB. The best way is to
> > do RF clipping followed by a band pass filter.
>
> The PM component can be ignored if the modulation bandwidth is very
> small compared to the RF frequency (Always the case in SSB), in which
> case the envelope is simply sqrt (I^2+ Q^2) which is trivial in a DSP
> based radio, and given we are only interested in the peaks, abs(audio)
> will come plenty close enough.
>
> > The problem is that the audio waveform is not the same as the RF
> > envelope waveform. It may look similar but operates entirely
> > different.
>
> Yep, the RF has a PM component, however the rf ENVELOPE is still
> very close to sqrt (I^2+Q^2) as long as the modulation bandwidth is very
> small compared to the RF frequency (It always is in our applications).
>
> > Put a square wave audio signal into an SSB transmitter any you only
> > get very large spikes out.
>
> You get spikes in the frequency domain of course, just the same as if
> you look at the frequency domain plot of the original square wave, but
> in the time domain you just get pulses of RF with some PM that you
> usually cannot see easily on a scope.
> Consider the fourier transform of a square wave and you will see what I
> mean.
>
> Compression followed by any number of clippers works fine, RF clipping
> is less audibly objectionable because most of the IMD falls outside the
> clippers filter, but a pair of diodes will work.
>
> 73 Dan.
>
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>
> On Mon, 2012-01-02 at 16:31 -0500, Gary Schafer wrote:
> > There is no easy way to do audio clipping for SSB. The best way is to
> do RF
> > clipping followed by a band pass filter.
> > The problem is that the audio waveform is not the same as the RF
> envelope
> > waveform. It may look similar but operates entirely different.
> >
> > Put a square wave audio signal into an SSB transmitter any you only
> get very
> > large spikes out.
> >
> > 73
> > Gary K4FMX
> >
> > >
> > > A clipper made from couple of diodes back to back across the audio
> line,
> > > followed by a lowpass filter will get you most of the way there,
> > > particularly if preceded by a compressor (THAT Corp do a very nice
> chip
> > > that can be trivially configured as a compressor/limiter/gate).
> > >
> > > The filter will of course introduce some overshoot and ringing, so a
> few
> > > stages may be indicated, but that is still only what a dozen or so
> > > components.
> > >
> > > Or a pair of SL670s or similar and a 445Khz ceramic filter plus a
> few
> > > diodes makes a dandy RF speech clipper/filter combo.
> > >
> > > I might draw one up and publish it somewhere.
> > >
> > > Personally I consider any ALC voltage to be an indication that
> something
> > > is not set up right, you know the amps power rating and power gain,
> so
> > > you know the required peak drive power, program that into the
> exciter
> > > and leave the ALC line unhooked (My exciter does NOT overshoot, some
> do,
> > > be careful).
> > >
> > > A reverse power sensor after the amp is a good idea if you go this
> way
> > > as the amp now has no way to fold back the exciter drive power.
> > >
> > > 73, Dan.
> > >
> > >
> > >
> > > _______________________________________________
> > > Amps mailing list
> > > Amps@contesting.com
> > > http://lists.contesting.com/mailman/listinfo/amps
> >
>
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